Translational regulation of a novel testis-specific RNF4 transcript.

The RING-finger protein SNURF/RNF4, a modulator of both steroid receptor dependent and basal transcription, is expressed at very high levels in testis and at much lower levels in several other tissues. In somatic tissues, the RNF4 gene is expressed as a 3-kb transcript while an additional shorter sized transcript (1.6 kb) was found in mouse testis. In murine germ cells, RNF4 protein expression is strongly modulated during progression of spermatogonia to spermatids, with a peak in spermatocytes. The expression of 3-kb transcript correlated with protein levels in the different germ cell populations. Conversely, the 1.6-kb transcript was abundantly and specifically expressed in spermatids, in which RNF4 protein was detected at very low levels. We have then examined possible mechanisms underlying this discrepancy. Primer extension and RNase protection analyses demonstrated that the 1.6- and 3.0-kb transcripts originate from the same promoter, encode for the same protein and differ in the 3' UTR. In vitro assays showed that protein degradation is not involved in the regulation of RNF4 protein level. Finally, polysome analysis revealed that only a slight fraction of the testis-specific transcript is engaged in translation, thus providing a feasible mechanism for the quantitative differences of RNF4 mRNA and protein levels. Present results demonstrate that RNF4 short transcript is poorly translated suggesting that this mechanism could be essential for normal spermatogenesis. Copyright 2003 Wiley-Liss, Inc.